Color mark detection is often required in manufacturing and packaging operations which monitor continuous products, such as monitoring lengths of paper/plastic material to be packaged on a roll. Mark detection requires differentiation of a colored mark from the colored background, and often only a few colors need to be recognized. This invention provides fast, single primary color recognition at low cost.
Chromatic aberration is a property of lenses which causes the various pure colors in a beam of polychromatic light to be focussed at different points due to the fact that the substance used for the lens refracts light of different pure colors by different amounts. In optical devices such as cameras, binoculars and the like, chromatic aberration is very undesirable and unwanted, and steps are taken to overcome or minimize the effect, for example by using achromatic lenses. In the present invention, by contrast, the chromatic aberration occurring in a gradient-index (GRIN) rod lens (sometimes known as a Selfoc lens) is desired. A key concept is the use of a cumulative chromatic aberration in an extended GRIN lens element to obtain primary color separation. The planar GRIN lens described herein provides substantial chromatic aberration and is readily obtained by lapping a conventional rod GRIN lens and terminating the length of the lens at the focal point plane of the desired primary color.